Michael Eckert1CS590SW: Web Ontology Language (OWL) Web Ontology Language (OWL) CS590SW: Semantic Web (Winter Quarter 2003) Presentation: Michael Eckert

Michael Eckert2CS590SW: Web Ontology Language (OWL) What is OWL? “The Web Ontology Language (OWL) is intended to provide a language that can be used to describe the classes and relations between them that are inherent in Web documents and applications” OWL can be used to  Formalize a domain by defining classes and properties of those classes  Define individuals and assert properties about them  Reason about these classes and individuals to the degree permitted by the formal semantics of the OWL language Motivation: “Computational Agents require machine-readable descriptions of the content and capabilities of web accessible resources. These descriptions must be in addition to the human-readable versions of the information.” “Ontologies facilitate greater machine readability of web content than XML, RDF, and RDF-S by providing additional vocabulary for term descriptions.”

Michael Eckert3CS590SW: Web Ontology Language (OWL) Three OWL languages:  OWL Lite: simplified language primarily for classification hierarchies and simple constraints  OWL DL: power corresponds to expressiveness of Descriptions Logics  OWL Full: a class can be treated simultaneously as a collection of individuals (extension) and as an individual in its own right (intension) W3C’s Architecture for a Semantic Web XML + NS + xmlschema UnicodeURI RDF + rdfschema Web Ontology Language (OWL)

Michael Eckert4CS590SW: Web Ontology Language (OWL)  Restriction of the expressiveness of First Order Logic (usually to the degree of PSPACE-completeness)  Semantic: set-theoretic interpretation  Concepts (Sets), Roles (binary Relations), Individuals (Elements)  Various Operators and Quantifiers  T(erminological)-Box: definitions and axioms  A(ssertional)-Box: information about individuals  Note: names of individuals can be used in a T-Box e.g.  open world assumption, no unique naming assumtion Foundation: Description Logics Classic DB-Systems First Order Logic Description Logics

Michael Eckert5CS590SW: Web Ontology Language (OWL) OWL: Basic Definitions Defining classes: Class Building class hierarchies: subClassOf Defining Individuals: Simple Properties: datatype properties: binary relation between elements of classes and XML datatypes object properties: binary relation between elements of two classes

Michael Eckert6CS590SW: Web Ontology Language (OWL) OWL: Complex Classes Form Classes using class expressions.  Set Operators intersectionOf, unionOf, complementOf  Enumerated Classes: oneOf Define a class via enumeration of its members  closes class extension  Disjoint Classes: disjointWith Set of mutually disjoint classes: one disjointWith assertion per pair

Michael Eckert7CS590SW: Web Ontology Language (OWL) OWL: Property Characteristics Are given in the property definition and apply globally to all instances of the property. Their specification provides a powerful mechanism for enhanced reasoning about a property.  Specification of domain and range  subPropertyOf  TransitiveProperty, SymmetricProperty  FunctionalProperty, InverseFunctionalProperty (“unique key”)  inverseOf OWL: Property Restrictions Are given in a class definition and apply only locally to their containg class.  Quantifiers: allValuesFrom, someValuesFrom  Cardinality: cardinality, minCardinality, maxCardinality  Relate individuals as particular property values to class: hasValue

Michael Eckert8CS590SW: Web Ontology Language (OWL) OWL: Reuse of Ontologies OWL supports RDF’s namespace mechanism and allows to importing other ontologies into the current knowledgebase via imports. To support effective sharing and reuse of ontologies we additionally need ways to identify correspondences between two ore more ontologies:  Equivalency: sameClassAs, samePropertyAs  Identity - Dealing with the no unique naming assumption: sameIndividualAs, differentIndividualFrom

Michael Eckert9CS590SW: Web Ontology Language (OWL) Summary  OWL builds upon XML and RDS  Three increasingly complex languages: OWL Lite, OWL DL, OWL Full  Theoretical foundation: Description Logics  Classes, Properties, Individuals  Reasoning: Property Characteristics, Restrictions, Complex classes  Reuse of Ontologies: Ontology Mapping Literature  W3C Sematic Web:  Web Ontology Language (OWL) Guide Version  Alexander Borgida: Description Logics in Datamanagement TKDE 7(5): (1995).